The present invention relates to an induction motor, and more particularly to a flat (disk-shaped) induction motor which has a disk-shaped stator and rotor placed coaxially with an air gap in the axial direction and is especially suitable for air conditioners.
A flat induction motor is a motor which has a disk-shaped stator and rotor placed coaxially around a rotating shaft with their surfaces opposing each other. Normally, the stator core and rotor core each have a spiral winding structure made of a magnetic steel strip. A plurality of open slots are formed in the winding structures from the outer edge toward the rotating shaft at equal intervals, leaving part of the magnetic steel strips.
Slots in the stator core are insulated and accept a main winding and auxiliary winding over the insulation while a secondary winding is put in the slots in the rotor core. Examples of such induction motors are described, for example, in Japanese Patent Application Publication No. S55-026029 and S59-113752.
In the induction motor with the above configuration, the auxiliary winding of the stator is connected in series with a capacitor while the main winding is connected in parallel with this series circuit. When an AC power supply is applied to this parallel circuit, a rotating magnetic field is generated in the stator, inducing a current in the secondary winding of the rotor. The interaction between the rotating magnetic field and induced current produces a torque which rotates the rotor.
Such induction motors are flatter and less bulky than radial-gap motors. Thus, they have the advantage of being able to reduce their installation space, and consequently, downsize the equipment such as air conditioners in which they are used.
However, the conventional induction motor with the above configuration, whose stator and rotor consist of winding structures made of magnetic steel plates, has the problem that it is difficult to fasten the stator to a bracket (casing), and the rotor to the rotating shaft.
This problem can be solved by integrally molding a resin bracket on the outer periphery of the stator. However, the use of resin is something to be avoided not only from the viewpoint of cost, but also from the viewpoint of environmental protection because of a waste-disposal problem.
Looking at the stator core and rotor core, the opening of the open slots is made wider to ease insertion of the windings. This tends to increase the reluctance (magnetic reluctance) of the air gap, which causes magnetomotive force to be wasted, resulting in decreased efficiency. Moreover, spatial variation of magnetic flux density in the air gap is increased, which may cause vibration and noise.
To solve this problem, the slot openings of the stator core can be made narrower, which, however, not only requires a dedicated winder, but also lowers the efficiency of winding operations. Similarly, if the slot openings of the rotor core are also made narrower, it becomes difficult to insert rotor windings in the slots.
Incidentally, as an alternative method for forming windings in the narrow slot openings, die casting may be adopted, but this has the problem of increased cost due to increase in the number of manufacturing processes.
Thus, an object of the present invention is to provide an induction motor which makes it possible to easily fasten the stator to the bracket, and the rotor to the rotating shaft, mount windings in the stator core and rotor core easily, maintain the efficiency of winding operations, reduce vibration and noise, and lower cost.
To achieve the above object, the present invention provides an induction motor which has a disk-shaped stator and rotor placed coaxially around a rotating shaft with their surfaces opposing each other across a predetermined gap and causes a rotating magnetic field generated in the windings of the stator to induce current in windings of the rotor, turning the rotor, wherein:
the stator includes a stator yoke and stator teeth, the stator yoke consists of a laminate made by laminating a plurality of blanked ring-shaped magnetic steel plates in the axial direction, holes for a predetermined number of slots are formed in the stator yoke at equal intervals in the circumferential direction, the stator teeth have first tooth bodies around which stator windings are wound and first tooth tips formed on those ends of the first tooth bodies which oppose the rotor, and the other ends of the first tooth bodies fit in the holes in the stator yoke;
the rotor includes a rotor yoke and rotor teeth, the rotor yoke consists of a laminate made by laminating, in the axial direction, a plurality of blanked disk-shaped magnetic steel plates with a hole for insertion of a rotating shaft at the center, holes for a predetermined number of slots are formed in the rotor yoke at equal intervals in the circumferential direction, the rotor teeth have second tooth bodies which are inserted as rotor windings into a conductor plate and second tooth tips formed on that end of the second tooth bodies which oppose the stator, and the other end of the second tooth bodies fit in the holes in the rotor yoke; and
the stator windings are fitted around the first tooth bodies between the stator yoke and first tooth tips, the rotor winding conductor plate is fitted around the second tooth bodies and sandwiched between the rotor yoke and second tooth tips, the stator has the outer edge of the stator yoke secured in a motor bracket, and the rotor is fastened to a rotating shaft held by bearings of the motor bracket.
The induction motor according to the present invention may be not only a single-stator, single-rotor type, but also a single-stator, double-rotor type. Specifically, the present invention has an aspect according to which the induction motor comprises two stators and rotors, the two stators are positioned back to back with each other in the center of the motor bracket, the two rotors are placed in opposing relation to the respective stators, the stator teeth of the two back-to-back stators are aligned with each other, and the windings around the back-to-back stator teeth differ in magnetic polarity from the respective rotors.
According to another aspect, the induction motor may be configured as a double-stator, single-rotor type, the two rotors being positioned back to back with each other in the center of the motor bracket, the two stators being placed in opposing relation to the respective rotors, the stator teeth of the two stators being aligned with each other, and the windings around the stator teeth of the stators differing in magnetic polarity from the respective rotors.
To decrease the failure rate and increase the reliability of the motor, preferably the backs of the stator teeth of the stators positioned back to back with each other or the backs of the rotor teeth of the rotors positioned back to back with each other are welded together by resistance welding.
Also, the stator yoke may be made of a non-magnetic, non-conductive material instead of the laminate of the magnetic steel plates. This will reduce magnetic flux leakage from the stator and improve motor efficiency.
In the present invention, preferably the conductor plate serving as the rotor windings is a metal plate or a laminate of metal plates with holes for accepting the second teeth bodies.
A conductive plate may be used for the rotor yoke itself instead of the magnetic steel plates and may combine the rotor winding conductor. A copper plate or aluminum plate is suitable for the conductive plate because it can reduce magnetic flux leakage from the stator and improve motor efficiency.
In the present invention, preferably the stator teeth and the rotor teeth are laminates made by laminating a plurality of magnetic steel plates in the radial direction of the respective yokes and the widths of the magnetic steel plates in the laminates in the circumferential direction of the respective yokes are narrowed gradually from the outer to the inner edge of the respective yokes. This will ensure an optimum magnetic circuit between the stator and rotor and improve motor efficiency.
Also, the ends of the stator teeth fitted in the stator yoke may be welded to the stator yoke while the ends of the rotor teeth fitted in the rotor yoke may be welded to the rotor yoke. This will increase the strength of the stator and rotor, eliminate rotor eccentricity, and increase the reliability of the motor.
Also, to make it easy and less expensive to fasten both stator and rotor, preferably the stator is fastened by press-fitting the stator yoke in the bracket and the rotor is fastened to the rotating shaft by shrinkage-fitting, press-fitting, or caulking the rotor yoke or the circular conductive plate which combines the rotor winding conductor over the rotating shaft.
Also, a reinforcement plate may be attached to a portion where the rotor is fastened to the rotating shaft to fasten the rotor securely at right angles to the rotating shaft, and thereby prevent eccentricity or run-out of the rotor.